Assisted-Global Positioning System (A-GPS) technology has emerged as a better alternative than conventional Global Positioning System (GPS) technology for providing accurate location determination for mobile terminals operating in wireless telecommunication networks. In an exemplary A-GPS system, a telecommunication network may transmit assistance data to a mobile device or terminal to reduce the search time for acquiring satellite signals. Advantages of A-GPS over conventional GPS include, among others, a reduced time-to-first-fix (TTFF), reduced battery consumption at the mobile device, and an increased GPS receiver sensitivity, thereby making it possible for the receiver to detect weaker satellite signals. There are two types of operation in the A-GPS scheme, mobile device based (MS-based) and MS-assisted modes. In the MS-based mode, a position determination entity (PDE) is situated inside the mobile device. In the MS-assisted mode, the mobile device measures satellite pseudo-ranges and passes these pseudo-ranges to a remote entity where the PDE resides. This remote entity may be a Serving Mobile Location Center (SMLC) for a Global System for Mobile Communications (GSM) network, a Radio Network Controller (RNC) or Stand Alone SMLC (SAS) for a Universal Mobile Telecommunications System (UMTS) network, and the like.
Conventionally, measurements from a minimum of five satellites are needed to perform a location determination in an A-GPS system as there are five unknowns in the problem including the three dimensional location of the mobile device to be located, user equipment (UE) clock bias, and GPS time bias. The minimum number of satellites to perform the location determination may be reduced to four if the elevation of the mobile terminal can be estimated based on apriori knowledge of the elevation of the cell serving the mobile terminal to thus introduce the earth's surface as an additional hyperboloid. Indoor environments and urban canyons, however, may prevent such an estimation where the mobile device may is unable to view four satellites. Therefore, there is a need in the art to reduce this requirement so that a location calculation may be performed with measurements from three satellites. There is also a need in the art to increase GPS receiver sensitivity when satellite visibility is limited.
The concept of using location measurement units (LMUs) for providing time information assistance to a GPS receiver has been discussed in U.S. Pat. No. 6,603,978 to Carlson, the entirety of which is incorporated herein by reference. Methods such as this may suffer from inaccurate estimates of the GPS-to-GSM time, or GPS-to-System time, and other timing relationships at the mobile device or terminal since propagation delay from the mobile terminal to the base-station/LMU may be ignored completely or approximated. Another technique, Multiple Range Estimation Location (MREL) described in U.S. patent application Ser. No. 12/292,821 and continuations thereof, the entirety of each being incorporated herein by reference, employs a method to accurately determine propagation delay. Other methods have depended upon a group of A-GPS capable mobile devices or terminals to build a database of GPS-to-System time relationships. US2008/0316091 to Wigren proposed such a database which may be used for fine-time assisted GPS techniques, however, such an approach adds overhead to the corresponding communications network to exchange assistance and response information to mobile devices. A further disadvantage for such a technique is for mobile devices in areas having low satellite visibility (e.g., urban canyons and indoor locations) it may not be possible to receive GPS signals thus making it difficult to construct the proposed GPS-System time database.